The above disclosed patents demonstrate that there are distinct advantages in coating pigments to produce a less dusty and better dispersed pigment product. The ideal colorant of this type must meet certain standards. The pigment particles must be non-dusting to comply with OSHA standards. The pigment must be highly dispersed so as to obtain the highest possible color strength of the resulting product. In addition, it is necessary to utilize an FDA-approved additive so that the resulting plastic color particle can be used in the plastic food container industry. It is desirable to obtain a high concentration of pigment particles in the matrix to produce the desired color at the lowest possible cost. Further, it is necessary to utilize equipment requiring the lowest possible energy and the highest possible combination of the desired results discussed above. The present invention relates to a method and composition made by this method which meet the above standards better than any product now known and/or used.
In accordance with the present invention, there is provided a method of preparing coated pigment particles for dry compounding with a polymeric plastic or rubber material, which method comprises the steps of providing particles of hydrophilic color pigment, mixing the pigment particles with a wax having a given melting point temperature and a brittle condition below a given lower temperature, heating the mixture above a process temperature substantially above the melting temperature and substantially above about 212.degree. F. to vaporize entrapped water, subjecting the mixture to high shear mixing while at or above the process temperature, cooling the mixture to a temperature below the given lower temperature to produce a solid mass, and, then, breaking the solid mass into flakes.
In accordance with another aspect of the present invention, there is provided a composition in the form of flakes, each of which includes color pigment particles each encapsulated in wax and the flakes having a water content of less than about 3%-4% by weight.
By using the above-defined invention, the resulting composition approaches an ideal colorant for use in polymeric plastic and rubber material. It is known that pigment agglomerations and aggregations are often produced in the form of pigment "presscakes". Presscakes are then dried to remove moisture and produce a relatively dry powder product. This dry product is then mixed with thermoplastic material and worked in an extruder. Thereafter, the extruded combination presscake and plastic material is chopped into pellets to form a commercial plastic stock. This is a hydrophilic type of material used as a colorant and incorporates the high moisture presscake. The presscake powder is mixed with the thermoplastic material to produce the colored plastic pellets. The extent of a dispersion of the presscake powder in the thermoplastic material is normally controlled by the amount of working done by the extruder used to extrude the plastic mass to break up the pigment agglomerates formed during the drying of the presscake. This prior system for producing a plastic colorant entraps water because of its hydrophilic nature and does not produce a finely dispersed pigment for subsequent coloring of plastic articles.
In the present invention, it is preferred to use presscake pigment before it has been dried. As is known, drying causes agglomerations due to the close natural bonding between the pigment after water is driven therefrom. Of course, dried pigment can also be used in accordance with the present invention. In the present invention, a high shear milling is employed as will be described in more detail. The special wax matrix contemplated by the present invention replaces the water binder of presscake particles with a wax binder to produce wax coated pigment particles. During the processing of the pigments, the normal water entrapped in the presscake particles is driven off by processing the mixture in an open milling container which allows rapid removal of water vapor from the pigment particles. The present invention reduces the amount of total energy consumed in the oven drying step which is normally used in drying presscake to remove its water content. In the present invention, the water content is removed simultaneously with the mixing operation. This saves a substantial amount of energy and creates a far superior end product. The direct replacement of water with a wax binder before the presscake pigment can agglomerate is a new concept in using presscake pigment. During the simultaneous driving off of water and incorporation of the special wax, which in practice is a hydroxystearate wax, pigments are protected against subsequent absorption of water. Thus, they are converted into a hydrophobic colorant having a long shelf life and which does not create striations and other defects in subsequently formed plastic due to retained or absorbed moisture. Thus, by using the combined water extraction and wax coating process, a substantial amount of energy is saved which is both economically and environmentally advantageous.
Another advantage of the present system is that the resulting colorant having a water content of less than about 3%-4% is extremely beneficial because of the reduction in water content, the hydrophobic nature of the compound, and the extremely fine dispersion of the pigments within the colorant to produce an increased hue in the resulting plastic article. In accordance with the present invention, a three roll mill having two heated feed rolls and an adjacent cooling roll are used. The present method can be performed by a single, commercially available piece of equipment with minor modifications. The machine contemplated by the present invention to perform the method set forth above produces a one step mechanical method for producing the finished chip or flakes of colorant which have the attributes of high pigment concentration, high color hue, and low moisture affinity. The resulting chip or flakes produced by the apparatus are non-dusting in that they do not break into dust particles which would cause difficulty in meeting the OSHA regulations for industrial manufacturing plants. The present invention produces a high dispersion characteristic due to the high shear mixing obtained in the milling machine contemplated by the present invention. The concept of making color concentrate or colorant in a single step using non-agglomerated presscake has not been used in the art of producing colorants from pigments. As previously mentioned, the term "presscake" relates to pigment particles which are formed in a manner to produce the pigment which is finely divided. This pigment has a high water content of approximately 50% and agglomeration can occur if this pigment is dried preparatory to use in a process for making a colorant from the pigment. In the past, if presscake were dried, it had to be subsequently milled to again produce unagglomerated pigment for subsequent processing. This required a substantial amount of energy and processing time not necessary in the present invention which can accept and process presscake pigment without prior drying. In this manner, presscake pigment remains non-agglomerated so that there can be a direct replacement of the moisture with wax during the mixing and heating process contemplated in the present invention. The wax matrix binders contemplated by the present invention are FDA approved for contact with food. In this manner, the resulting plastic material can be formed into plastic containers for use in the food process industry which is an important feature in any colorant for plastic material. If the colorant does not have FDA approval or is formed from material not having FDA approval, this is a serious economic and practical drawback to the colorant. The present invention also contemplates the use of FDA approved "FD & C pigments". These color pigments are produced and are commercially available in presscake or other form containing generally 5-30% water. This type of FDA-approved pigment can be used in the present invention without dehydration and the resulting composition or colorant has a final water content of less than 3% by weight. Thus, the resulting product is acceptable to the FDA and has a low moisture content which is necessary to produce uniform, high-luster colors in plastic and rubber materials. The high pigment loading obtained by the present invention is in excess of approximately 75-90% by weight which is apparently obtainable because of the use of the milling process which allows a thorough heating of the special wax and pigment to drive off the water and immediately replace the water with wax within a high shear mixing arrangement which enhances the amount of pigment that can be loaded into a selected amount of wax. In the present invention, an open milling arrangement is used instead of a confined fixed tank or closed type of mixing unit which hinders the extraction of liquid and produces a low shear type of mixing which reduces the amount of pigment loading. Used in the present invention, presscake pigment which has a low dusting factor can be directly introduced into the processing machinery. This prevents loss of pigment from the dusting phenomenon. The resulting pigment does not form into individual particles until it is mixed in the heating and high shear mixer which liquefies the special wax matrix, replaces the water driven off and encapsulates the individual color pigment particles before they can escape into the atmosphere. Thus, the wax binds the product into the mixer until a wax-pigment saturation mixture is obtained. This mixture is then passed over a cooling roll which chills the mixture to a brittle state on the roll ifself. The solid mixture is then chipped from the roll to produce snowflake-like chips or flakes which is the resultant commerical product to which the present invention is directed. It can be seen that a very simple, inexpensive, low energy consuming apparatus can be used for producing colorant flakes with high pigment dispersion and low moisture content and affinity. This concept has not been used or contemplated in the art of making colorants for plastic or rubber material prior to the development of the present invention.
The object of the present invention is to satisfy the criteria set forth in the above section. As somewhat of a summary, one of the primary objects of the present invention is the provision of a method of preparing coated pigment particles for dry compounding with plastic or rubber material, which method produces a colorant having a low moisture content.
Another object of the present invention is the provision of a method as defined above which can be performed on inexpensive equipment using relatively small amounts of energy.
A still further object of the present invention is the provision of a method as defined above, which method produces a colorant having a high concentration of pigment in a wax matrix.
Still a further object of the present invention is the provision of a method as defined above, which method produces a colorant having a high concentration of pigment, evenly dispersed in a wax matrix.
Yet another object of the present invention is the provision of a method as defined above, which method produces a colorant that has a low affinity for moisture, a long shelf life and uses a wax substantially less expensive than a fatty acid amine wax.
Still a further object of the invention is the provision of a method as defined above, which method uses high shear in mixing pigment particles with a hydroxystearate wax.
Another object of the present invention is the provision of a new plastic colorant, which colorant includes flakes of pigment and hydroxystearate wax with a moisture content of less than about 3%-4% by weight, a high pigment concentration in the wax matrix and a fine, even dispersion of the pigment within the matrix.
These and other objects and advantages will become apparent from the description of the present invention which is configured hereinafter.